feat: implement event hub for sensor data collection

## Description
Build the event hub that collects and stores sensor events from edge agents.

## Requirements
- [ ] POST /fleet/events: accept event payload (node_id, type, data, timestamp)
- [ ] Event validation and normalization
- [ ] In-memory ring buffer (configurable size)
- [ ] Query API: GET /fleet/events?node_id=X&type=Y&since=Z

## Skills
Go or TypeScript, event processing